1. Field of the Invention
The present invention relates to a toner for developing electrostatic images used for copier, printers, and facsimile machines. The present invention also relates to a developer, a toner container, a process cartridge, an image forming apparatus, and an image forming method using the toner.
2. Discussion of the Background
As methods of fixing toner on a recoding sheet, contact-heating fixing methods such as methods using a heating roller (hereinafter “heating roller fixing methods”) are widely employed. Typically, fixing devices employing heating roller fixing methods contain a heating roller and a pressing roller. Recording sheets having toner images thereon are passed through a nip formed between the heating roller and the pressing roller so that the toner melts and is fixed on the recording sheet.
In contact-heating fixing methods such as heating roller fixing methods described above, a toner image on a recording sheet may directly contact a surface of a heating member (such as the heating roller) of a fixing device. Therefore, a part of the toner image may adhere to the heating member and may be re-transferred onto an undesired portion of a next recording sheet. This phenomenon is hereinafter referred to as “offset phenomenon”.
To prevent the occurrence of offset phenomenon, one proposed approach involves coating or impregnating heating and pressing rollers with oils such as silicone oils. However, this approach disadvantageously causes upsizing of fixing devices and cost increase due to provision of an oil applicator. In view of such a situation, fixing devices containing no oil applicator or requiring lower amounts of oils are employed recently. Such fixing devices typically use toners containing a releasing agent that serves as an offset inhibitor.
From the viewpoint of energy saving, the heating temperature of the heating member is preferably set as low as possible. Accordingly, thermal properties of binder resins composing toners are preferably as low as possible so that the binder resins can melt at lower temperatures. However, if thermal properties of binder resins are too low, heat-resistant storage stability may deteriorate, which may case toner blocking. Polyester resins, which typically have lower viscosity and higher elasticity compared to vinyl copolymer resins, are advantageously used for binder resins because they melt at lower temperatures and have good heat-resistant storage stability.
When a toner containing a sufficient amount of a release agent is produced by a conventional pulverization method, the release agent tends to expose at surfaces of resultant toner particles. As a result, toner blocking or a filming problem in which undesired toner films are formed on image forming members may occur. On the other hand, such a toner can be produced by polymerization methods such as a suspension polymerization method in which a polymerizable monomer is subjected to a polymerization in an aqueous medium to form toner particles and an emulsion aggregation method in which fine particles formed by an emulsion polymerization are aggregated to form toner particles. Polymerization methods are typically capable of including greater amounts of release agents in resultant toners compared to pulverization methods. Japanese Patent No. 3195362 discloses a suspension polymerization toner, the configuration of which is controlled. Specifically, toner particles formed by a normal suspension polymerization are subsequently subjected to an extra polymerization with additional monomers. Japanese Patent No. 3994697 discloses an emulsion aggregation toner, the configuration of which is also controlled. Specifically, toner particles formed by a normal aggregation are subsequently subjected to an extra aggregation with additional emulsion polymerization particles. These toners are composed of vinyl copolymer resins because both suspension polymerization methods and emulsion aggregation methods are capable of forming vinyl copolymer resins in an aqueous medium. Unlike vinyl copolymer resins, polyester resins are difficult to be formed by either suspension polymerization method or emulsion aggregation method because they are typically polymerized at a high temperature of 200° C.
One possible toner manufacturing method to use polyester resins is a so-called dissolution suspension method in which toner components including a resin are dissolved in an organic solvent and the resultant toner components solution is subjected to granulation in an aqueous medium. Because the resin is never subjected to polymerization in this method, the molecular weight of the resultant toner equals to that of the raw-material resin. Therefore, if thermal properties of the resultant toner need controlling, a low-molecular-weight resin and a high-molecular-weight resin may be mixed in the toner components solution in advance. However, if the amount of the high-molecular-weight resin is too large, the toner components solution may have too large a viscosity, resulting in deterioration of granulation efficiency. Accordingly, the amount of the high-molecular-weight resin is preferably minimized. Instead, the molecular weight of the low-molecular-weight resin needs to increase to some extent, which is disadvantageous for fixing the resultant toner at low temperatures.
To solve this problem, one proposed approach involves elongating and/or cross-linking a modified polyester which has a reactive group after granulation of toner particles, instead of mixing a high-molecular-weight resin in raw materials, to control the molecular weight of the resultant toner. This approach has an advantage in controlling thermal properties of toner, but has a disadvantage in controlling the structure of toner.
As described above, most of recent electrophotographic image forming apparatuses use toners containing a release agent such as a wax. Toners are generally required to express an appropriate gloss when formed into images. It is known that human eyes have a preferable range of gloss. In a case in which the gloss is beyond the preferable range, we may feel uncomfortable sensation, especially when the image is in full-color.
Waxes make a great effect on the gloss of resultant toner images. The greater the amount of wax in toner, the lower the resultant image gloss, and vice versa. This is because waxes in the resultant images cause diffuse reflection of light. Since the gloss is generated based on regular reflection of light, increase of diffuse reflection reduces regular reflection, resulting in deterioration of the gloss. In a case in which the amount of wax in toner is unstable, the gloss may be unstable as well.
In pulverization methods, undesired ultra-fine particles are produced at a time a raw material mixture is pulverized. This is because the raw material mixture easily splits from interfaces of waxes and resins. Therefore, the ultra-fine particles generally include a large amount of wax, but the amount of wax varies depending on production lot of toner. As a result, the gloss of the resultant image varies depending on production lot of toner as well, which causes unreliable image forming.
Accordingly, with regard to toners containing a binder resin and a release agent (such as a wax), no toner is provided which can reliably form high-gloss images.